1. Field of the Invention
The present invention relates to a frequency converting system for waveform-analyzing a high-frequency and broadband analog signal utilized in communication such as mobile communication, and the like or converting a frequency for a digital analysis device for spectrum-analyzing the analog signal and, more particularly, to a frequency converting system capable of reducing distortion caused in a converted wave resulting from instantaneous power contained in a broadband analog signal, and realizing a broadband converted wave of a high dynamic range.
2. Description of the Related Art
A wireless signal utilized in communication such as mobile communication is a modulated wave in a high-frequency band of several GHz or more, and a wide band of several tens of MHz or higher. As an apparatus for carrying out analysis such as spectrum analysis of a signal in such a high-frequency band and wide band, an analog spectrum analyzer has conventionally been used.
It is known that the analog spectrum analyzer includes a circuit configuration as shown in FIG. 15 as disclosed in JP-A H02-47563 (KOKAI)). In the spectrum analyzer, an analog signal x(t) to be measured is input to a mixer 12 to which a local signal from a local signal generator 11 is supplied. The analog signal x(t) is mixed with the local signal L in the mixer 12, and a mixing signal is output from the mixer 12 to a band-pass filter 13 of a narrow band. In this band-pass filter 13, a difference frequency component between the analog signal x(t) and local signal L is extracted from the mixing signal, and the extracted signal is output therefrom. Frequencies of the local signal L are swept in the wide band. Concomitantly with the sweep of the frequencies, the level of each frequency component contained in the analog signal x(t) is detected from the output signal from the band-pass filter 13, and the waveform or spectrum of the analog signal x(t) is analyzed.
However, when the analog signal x(t) to be measured is the broadband modulated wave as described above, peak power of a very large ratio of peak power to average power is contained in the analog signal x(t) and, in the spectrum analyzer described above, all the power of the analog signal x(t) is input to the mixer 12. Accordingly, such an analog signal x(t) causes inter-modulation distortion to occur in the output signal of the mixer beyond the appropriate operation range of the mixer 12, and causes an error to occur in the measurement result.
In order to prevent the inter-modulation distortion from occurring, normally an attenuator is provided in the preceding stage of the mixer 12, and a circuit configuration for attenuating the peak power of the input signal in such a manner that the peak power of the input signal becomes in the appropriate range is employed. However, in this circuit configuration, there is the problem that the level of the average power input to the mixer 12 is lowered by the attenuator, consequently, the S/N ratio of the signal is lowered, and the measurement dynamic range becomes narrow.
Further, as a circuit system for solving this problem, there is a filter selection system in which a plurality of filters different from each other in the band are provided in the preceding stage of the mixer 12 so that the filters can be selected, and a filter in the preceding stage is selected in accordance with the frequency of the local signal L supplied to the mixer 12. Further, as another circuit system, there is a tracking system in which a filter of a variable-frequency type is provided in the preceding stage of the mixer 12, and the frequency of the filter is changed to follow the frequency of the local signal. However, each of these systems is a system in which the spectrum of the input signal is analyzed by the sweep of the local signal L, and hence there is the problem that the analog signal x(t) cannot be theoretically measured in real time.